1. Field of the Invention
The present invention relates to the field of intravascular deployment systems. More particularly, the invention relates to the field of apparatus and methods for the deployment of intravascular devices, including exclusion devices such as stent grafts, where interference between the components of the delivery apparatus may lead to an inability to deploy the device or result in partial deployment of the device.
2. Background of the Art
Intravascular deployment of exclusion devices (stent grafts) is a methodology used to deliver an exclusion device to a portion of a body flow lumen that is diseased or damaged, such as an aneurysmal portion of an aorta, and, thence deploy the exclusion device to span the diseased or damaged portion of the aorta and thereby provide a synthetic flow conduit which passes through the diseased or damaged portion of the aorta and seals against healthy tissue of the aorta at locations upstream and downstream of the diseased or damaged portion thereof. By deploying the exclusion device intravascularly, the diseased or damaged portion of the flow lumen may be bypassed with the exclusion device forming a synthetic flow lumen, without the need to remove the diseased or damaged portion of the flow lumen, which would require far more invasive surgery. Where the diseased or damaged flow lumen is the aorta, either the abdominal aorta or thoracic aorta, the use of intravascular deployment of a stent graft to exclude the diseased or damaged portion of the aorta, and provide a secondary flow conduit within the aorta, is well known to those skilled in the art.
A stent graft includes a stent portion or frame, which is in some embodiments configured as a plurality of wires formed into hoops, to which is affixed a graft material, which is likewise formed into a hoop shape to provide a synthetic flow conduit for blood once the stent graft is deployed in the aorta. The stent graft is sized to have a length sufficient to span the diseased or damaged portion of the aorta, and overlap 10 to 25 mm onto the adjacent healthy tissue and a diameter one of two millimeters larger than the diameter of the healthy portion of the aorta located on the upstream and downstream ends, in a blood flow direction perspective, of the diseased or damaged portion of the aorta, such that the stent portion biases the graft material against healthy aorta wall tissue at the upstream and downstream ends of the stent graft to seal off the disease or damaged aorta wall from further blood flow thereto at systemic pressure.
To enable intravascular deployment of the stent graft, the stent graft is first radially compressed to a small diameter, on the order of a centimeter or less, and loaded into a tubular element, specifically a graft cover portion of a deployment system. The deployment system, includes the tubular graft cover, within which the compressed stent graft is inserted at the distal end thereof, a manipulator or middle member within the stent graft extends through the graft cover to the proximal end, thereof, and a guidewire can, extend through a bore in the middle member which extends the length of middle member and through the compressed stent graft, such that a first end of the guidewire can be disposed beyond the proximal end of the graft cover, and a second end is extendable from the bore at the distal end of the graft cover. The middle member serves several purposes: It provides the bore through which the guidewire is received such that the middle member, and the graft cover and stent graft thereover; may be tracked over the guidewire; it provides a support or “stent stop” against which the stent graft will bear during the deployment of the stent graft procedure; during the procedure the graft cover is retracted from around the stent graft and middle member; and, it provides, in conjunction with the graft cover, support or structure to carry the axial, rotational and bending loads imposed upon the delivery system as it is tracked over the guidewire.
Endovascular delivery of a stent graft is commonly facilitated by opening an incision into one of the iliac arteries adjacent the groin of the patient, and first deploying the guidewire, having fluoroscopic markers adjacent to the distal or deployed, end thereof, through or along the artery to a position wherein the distal end of the guidewire extends beyond the diseased portion of the aorta. The stent graft delivery catheter having the graft cover, having the middle member and the stent graft held therein, is then tracked along the guidewire, such that the distal end of the graft cover is positioned upstream of the deployment location of the stent graft. The distal end of the graft cover is then exposed to the aorta, and the graft cover is retracted while the middle member is held stationary, such that the stent graft cannot move relative to the stationary stent stop and the stent graft becomes exposed to the aorta and is deployed from the graft cover.
One issue which may arise during deployment of the stent graft from the graft cover, and which has serious consequences, is that the graft cover may become bound up with the stent stop, such that the graft cover cannot be retracted or moved relative to the stent stop. One cause of this binding is buckling of the graft cover, which can occur when the graft cover and middle member are being tracked along the guidewire through regions of tortuous anatomy. Because the graft cover is a thin walled tubular column, which is being pushed through restricted or tortuous pathways of an artery to reach the diseased portion of the aorta, forces may be imposed axially, i.e., the pushing of the graft cover from its proximal end as it is being tracked over the guidewire, rotationally, by forces imposed on the graft cover as the surgeon or other practitioner rotates the proximal end of the graft cover to properly align the stent graft at the deployment location, and in bending, by forces which are imposed as the delivery system is tracked through turns or restrictions in the introduction artery or the aorta. Turns result in the delivery system having one portion of the delivery system positioned in a generally linear path which is at an angle to the immediately adjacent portions of the delivery catheter. If the sum of these forces or loads exceeds the buckling strength or capacity of the graft cover, i.e., its resistance to excess deformation, then the graft cover can buckle. When such buckling occurs, the span across the interior of the graft cover is reduced at the buckle. If this occurs in the region of the graft cover extending about the stent graft held within the graft cover, as the graft cover is retracted to deploy the stent graft, the cover can become bound against (create an interference fit with) the enlarged portion of the middle member which forms the stent stop, preventing further retraction of the graft cover. Where the buckle interferes with the stent stop before substantial deployment of the stent graft, this is an inconvenience, as the procedure must be terminated and the delivery system with the stent graft intact, must be removed from the body by reverse tracking thereof over the guidewire. Where the buckle is brought against the stent stop after a portion of the stent graft is deployed, and the surgeon cannot pull the graft cover further over the middle member, immediate emergency surgery, to open the patient through the chest and invasively repair the situation is warranted.